This application claims priority of Korea patent Application No. 10-2000-71894, filed on Nov. 30, 2000.
(a) Field of the Invention
The present invention relates to a creep-control method for an automatic transmission of a vehicle, and more particularly, to a creep-control method wherein a second shift-speed is maintained to prevent rearward rolling on a slope and shift shock is reduced by optimal shift control from the second shift-speed to a target shift-speed when a creep-control state is released.
(b) Description of the Related Art
A conventional automatic transmission, equipped with a torque converter, performs shift control based on a variety of inputs regarding a driving state of a vehicle.
FIG. 1 shows a block diagram of a control apparatus for a conventional automatic transmission.
As shown in FIG. 1, the control apparatus includes a driving-state detecting unit 10 for detecting a plurality of vehicle driving-state factors, a transmission control unit (referred to as xe2x80x9cTCUxe2x80x9d hereinafter) 20 for determining an optimal shift pattern for the detected driving state and for controlling hydraulic shifting according to the determined optimal shift pattern, and an actuator unit 30 for performing hydraulic shifting according to the control of the TCU 20.
The driving-state detecting unit 10 includes a throttle valve open-angle detector 11 for detecting an open-angle of the throttle valve, a vehicle speed detector 12 for detecting a speed of the vehicle, a shift-lever position detector 13 for detecting a position of the shift lever, a turbine-speed detector 14 for detecting a revolution speed of the turbine in a torque converter (not shown), and an engine-speed detector 15 for detecting a revolution speed of the engine.
When the shift-lever is in a forward range such as a drive xe2x80x9cDxe2x80x9d range, a second xe2x80x9c2xe2x80x9d range, and a low xe2x80x9cLxe2x80x9d range, rotational torque of the engine is transmitted to an output shaft of the automatic transmission even if the vehicle is stopped because the transmission maintains a predetermined shift-speed. The control of the transmission to maintain the predetermined shift-speed at a very low vehicle speed is called creep-control, and the phenomenon whereby rotational torque is transmitted to the output shaft under creep-control is called a creep phenomenon. The predetermined shift-speed can be either of the first shift-speed and the second shift-speed.
FIGS. 2a, 2b, 2c, 3a, and 3b are speed diagrams for a conventional four-speed automatic transmission, wherein FIG. 2a shows a state in which the vehicle is driven in a first shift-speed, and FIG. 3a shows a state in which the vehicle is driven in a second shift-speed.
A shift mechanism of the transmission receives engine torque via a turbine of a torque converter through a rear clutch in the first and second shift-speeds. Therefore, the revolution speed of the rear clutch in the first and second shift-speeds can be understood as the revolution speed of an input element of the shift mechanism in those shift-speeds.
In the second shift-speed where a kick-down band brake is operated, a first operating element N1 as shown in the far right of FIG. 3a is stopped.
In the first shift-speed, the rear clutch N4 and the first operating element N1 revolve at opposite sides of a one-way clutch OWC.
More particularly, the first operating element N1 rotates in an opposite direction to the rotation of the rear clutch N4, the rear clutch N4 acting as an input element in the first shift-speed, because a second operating element on which the one-way clutch OWC acts is prevented from rotating backward.
Therefore, in a normal driving state, the rotational speed of the input element of the rear clutch N4 is changed according to the speed diagram of FIG. 2a and is transmitted to an output element of a third operating element N3.
However, when the vehicle is stopped, the output element of the third operating element N3 is also stopped. Therefore, the rear clutch N4, engaged with the third operating element through a gear mechanism, is also stopped. Accordingly, revolution speeds of each of the first, second, third, and fourth operating elements N1, N2, N3, and N4 are as shown in FIG. 2b. 
A difference between the engine speed and the revolution speed of the rear clutch N4 implies that slip corresponding to the speed difference occurs in the torque converter.
The torque converter transmits engine torque to the shift mechanism when slip occurs, and accordingly the shift mechanism of the automatic transmission changes revolution speed and torque of the engine, and outputs the changed speed and torque through the output element. Therefore, the output torque acts as a driving torque of the vehicle.
The driving torque of the vehicle acts as a driving force to drive the vehicle on a plane even when an accelerator pedal is not depressed, and it also acts as a force that prevents rearward rolling when the vehicle is stopped on a slope.
However, if the driving torque output from the output element is not sufficient, the vehicle may roll rearward on a steep slope when a driver takes his/her foot from a brake pedal.
FIG. 2c shows how the speed diagram may be changed when a vehicle is stopped on a slope and a brake pedal is released.
Gravitational force acts rearward on the vehicle when the vehicle is stopped on a slope and a brake pedal is released. However, the second operating element N2 cannot have a negative revolution speed rate because of the one-way clutch OWC if the shift mechanism of the automatic transmission is set to the first shift-speed.
Therefore, if the torque transmitted to the shift mechanism is not sufficient, the speed diagram can pivot anti-clockwise around the center of the second operating element N2, and accordingly the output element N3 can turn backward, which implies that the vehicle rolls rearward.
In the prior art in which an idle revolution speed of the engine is preset to be high, the problem of rearward vehicle rolling is not significant because of high torque transmitted to the shift mechanism. However, in the recent progression of engine control methods, it has been a trend that idle rpm of the engine is controlled as low as possible because it lowers fuel consumption. Therefore, rearward rolling of a vehicle on a slope has become more problematic in recently produced vehicles.
The prior art in which the shift mechanism of an automatic transmission is maintained at a first shift-speed also causes inconvenience in that a brake pedal must be pushed with a high force to keep the vehicle stationary.
The problem of rearward rolling on a slope and the inconvenience of applying the high braking force needed to hold the vehicle stationary according to the prior art can be solved by maintaining the shift mechanism at a second shift-speed for creep-control, which is herein-after explained in detail with reference to FIG. 3b. 
In the second shift-speed, the first operating element N1 is stopped and the second operating element N2 is prevented from turning backward because of the one-way clutch OWC.
Therefore, the point of the output element N3 in the speed diagram cannot be lowered below 0 even in the case when gravitational force acts as a rearward rolling force on the vehicle on a slope, because the speed diagram has a fixed point of the first operating element N1, and the second operating element N2 cannot turn backward.
However, maintaining the shift mechanism in the second shift-speed may cause another problem.
When the accelerator pedal is depressed, the shift mechanism must be transformed into a first shift-speed from the second shift-speed. However, sudden release of the first operating element N1 may cause a shift shock resulting from a sudden stop of the second operating element by the function of the one-way clutch OWC because the vehicle, and more particularly the second operating element N2, may gain a certain amount of speed during the time between releasing a brake pedal and depressing an accelerator pedal.
The present invention has been made in an effort to solve the problems and inconvenience of creep-control methods of the prior art.
It is an objective of the present invention to provide a creep-control method for an automatic transmission of a vehicle wherein a second shift-speed is maintained to prevent rearward rolling on a slope and shift shock is reduced by optimal control of hydraulic pressure when the creep-control state is released.
To achieve the above objective, the present invention provides a creep-control method for an automatic transmission of a vehicle comprising:
creep-entering, in which a hydraulic circuit of the automatic transmission is controlled to form hydraulic pressure supply lines of a second shift-speed when a predetermined creep-entrance condition is satisfied;
creep-maintaining, in which the formed hydraulic pressure supply lines of the second shift-speed are maintained and a hydraulic supply pressure of the formed hydraulic pressure supply lines is maintained at a predetermined creep duty ratio; and
creep-escaping, in which one of controlling the hydraulic circuit to engage a second shift-speed shift mechanism of the automatic transmission and controlling the hydraulic circuit to engage a first shift-speed shift mechanism of the automatic transmission is selectively performed based on a vehicle speed and throttle valve open-angle when a predetermined creep-escape condition is satisfied.
The creep-entrance condition is calculated based on a logical multiplication of a shift lever of the automatic transmission being disposed at a forward drive range position except a low xe2x80x9cLxe2x80x9d range, the vehicle speed being less than a first predetermined speed, the throttle valve open-angle being less than a first predetermined open-angle, and a speed shift based on a predetermined shift pattern not being required.
The predetermined creep duty ratio in the creep-maintaining is higher than 0 and lower than its maximum duty ratio.
The creep-escape condition is calculated based on a logical sum of the shift lever of the automatic transmission being disposed at a position of low xe2x80x9cLxe2x80x9d, park xe2x80x9cPxe2x80x9d, reverse xe2x80x9cRxe2x80x9d, and neutral xe2x80x9cNxe2x80x9d ranges, the vehicle speed being higher than a second predetermined speed, the throttle valve open-angle being greater than a second predetermined open-angle, and a shift-speed of the automatic transmission being required to be maintained at the second shift-speed.
The second predetermined speed is higher than the first predetermined speed and also higher than a threshold speed for a 2xe2x86x921 speed shift according to a predetermined 2xe2x86x921 shift pattern, and the second predetermined open-angle is greater than the first predetermined open-angle.
In the creep-escaping, the controlling the hydraulic circuit to engage the second shift-speed shift mechanism of the automatic transmission is performed when at least one condition of the vehicle speed being higher than a second predetermined speed, and a shift-speed of the automatic transmission being required to be maintained at the second shift-speed is satisfied, wherein the controlling the hydraulic circuit to engage the second shift-speed shift mechanism of the automatic transmission comprises performing resident steps of a predetermined 1xe2x86x922 speed shift control method, the resident steps being determined on the basis of the vehicle speed and the creep duty ratio for the hydraulic supply pressure of the formed hydraulic pressure supply lines of the second shift-speed.
In the creep-escaping, the controlling the hydraulic circuit to engage the first shift-speed shift mechanism of the automatic transmission is performed under the condition of the throttle valve open-angle being greater than the second predetermined open-angle, wherein the controlling the hydraulic circuit to engage the first shift-speed shift mechanism of the automatic transmission comprises:
duty ratio adjusting, in which a duty ratio for the hydraulic supply pressure of the hydraulic pressure supply lines of the second shift-speed is suddenly lowered from the creep duty ratio by a predetermined adjust value;
first ramp-controlling, in which the duty ratio for the hydraulic supply pressure is gradually lowered until a turbine-rpm is increased by a predetermined rpm;
duty ratio holding, in which the duty ratio for the hydraulic supply pressure is suddenly increased by a predetermined ratio and the increased duty ratio is held for a predetermined duration if the turbine-rpm is increased by the predetermined rpm in the first ramp-controlling;
second ramp-controlling, in which the duty ratio for the hydraulic supply pressure is gradually lowered until the turbine-rpm becomes within a predetermined range from a target rpm of the first shift-speed; and
transforming the hydraulic pressure supply lines to be hydraulic pressure supply lines of the first shift-speed.
The predetermined adjust value by which the duty ratio is suddenly lowered in the duty ratio adjusting is calculated by an increasing function of each of vehicle speed and throttle valve open-angle.
The predetermined ratio by which the duty ratio is suddenly increased in the duty ratio holding is calculated by an increasing function of vehicle speed.
At least one of the first ramp-controlling, duty ratio holding, and second ramp-controlling is performed within a predetermined period of time.